1. Field of the Invention
This invention generally relates to electronic control units applicable to vehicles, and more particularly to a failsafe process.
2. Description of the Related Art
In recent years, an increased number of electrical components on the vehicle has been used to improve vehicle safety and conformability, and a lot of more complicated vehicle controls has been needed. There is an increased number of signals supplied to electronic control units from sensors and switches for the purpose of various vehicle controls.
Each electronic control unit is equipped with a processor, which is capable of receiving a limited number of input signals. Therefore, one or more multiplexer circuits are provided at the input interface of the electronic control unit in order to handle more input signals. This kind of technique is disclosed in the following documents: Japanese Patent Application Publication No. 8-22308; Japanese Patent Application Publication No. 2-238379; Japanese Patent Application Publication No. 2-287744; and Japanese Patent No. 3202382.
A failsafe process should be executed to ensure the maximum safety if an abnormality occurs in any of the signals from the sensors and switches applied to the on-vehicle electronic control unit, particularly, in any of signals directly involved in travel control of vehicle. However, the conventional failsafe process has the following problem. If the input interface becomes faulty, the electronic control unit cannot discriminate abnormality in the input interface from that in the signals from the sensors and switches. This prevents reliable execution of the failsafe process.
The multiplexer circuits in the input interface may always be monitored in order to detect abnormality. This monitoring should be carried out by the electronic control unit when each multiplexer circuit is idle. This increases the load of the electronic control unit.